1. Field of the Invention
The present invention relates to a gear and a differential apparatus provided therewith for a vehicle, and more particularly relates to the gear having two gear portions and the differential apparatus for a vehicle disposed between a right and a left tire wheels or between front tire wheels and rear tire wheels.
2. Description of the Related Art
A differential apparatus for a vehicle is disclosed in Japanese Tokkaihei No. 8-247253, especially in FIG. 2 which disclosing a plurality of pinion gears and side gears, whose axes are parallel to each other.
This conventional differential apparatus for a vehicle as prior art mainly consists of a differential case rotated by receiving torque from an engine, a pair of side gears, each disposed on a rotational axis O of the differential case, a first pinion gear intermeshing with one of the pair of the side gears and a second pinion gear intermeshing with the first pinion gear and the other of the pair of the side gears respectively. The second pinion gear includes two gear portions.
The differential case has a pair of tire shaft inserting in which two tire shafts are respectively inserted. A gear accommodating space in the differential case accommodates the pair of side gears, the first pinion gear and the second pinion gear.
The pair of side gears accommodated in the gear accommodating space are positioned on the rotational axis of the differential case. One of the pair of side gears has a smaller pitch diameter than the other side gear. The tire shafts are respectively connected by spline fitting to an inner surface of the pair of side gears.
The first pinion gear is positioned at a periphery of the one side gear having the smaller pitch diameter.
The second side gear is a step gear having a step or neck portion between two gear portions. One gear portion having the smaller pitch diameter is disposed to engage with the first pinion gear and with the one of the pair of side gears, and the other gear portion having the larger pitch diameter is disposed to engage with the other side gear of the pair of side gears.
When torque from the engine of the vehicle is input to the differential case, the rotational torque is transmitted to the first pinion gear and the second pinion gear. Because the pair of side gears are engaged with the tire shafts, torque from the engine is distributed according to a driving state of the vehicle, to be transmitted to the right and left tire shafts through the differential case, the first pinion gear, the second pinion gear and the pair of side gears.
In this case, when the first pinion gear and the second pinion gear tend to rotate around their own axes upon a differential between the side gears, this causes sliding of the first and second pinion gears on inner surfaces defining the gear accommodating space of the differential case, thereby generating frictional resistance between the first/second gears and the inner surfaces to restrict the differential rotation of the pair of side gears.
Also, although there is no explicit disclosure in this document, it is obvious for an ordinary skilled person in this technology that where each of the gears is constructed as a helical gear and a thrust force is generated at an intermeshing portion upon a transmission of torque from the first pinion gear and the second pinion gear to the pair of side gears, the pair of side gears are moved along their axis by the thrust force. Then, the pair of side gears are pressed to each other or to the inner opening peripheral edges of the tire shaft inserting holes so that rotational frictional resistance is generated between the pair of side gears.
However, because there is a step portion, the mechanical strength of the second pinion gear against torsion torque generated at the step portion is weak. Therefore, in order to maintain strength of the second pinion gear against torsion torque, the diameter of the second pinion gear, or the set number of pinion gears, is made larger. As a result, the apparatus becomes large when the diameter of the gear is large or the total cost of the apparatus becomes high when the set number is large.
Also, where the conventional differential apparatus is disposed between the right tire shaft and the left tire shaft, a strictly identical differential rotational ratio or differential restricting performance between the pair of side gears connecting right and left tire shafts is required, as compared to that between the front and rear tire shafts. However, it is difficult to achieve the exact same differential rotational ratio or differential restricting performance, whereby there can occur different torque transmitting performances for the right and left tire shafts during straight driving or an unbalanced differential rotational ratio or differential restricting performance during right or left turning.